test suite: protocol-compat: check that we use the right distro

* Set distro to '' when running the import.  This makes dgit fall back
  to the default distro.

* Set the default distro to `foreign' (which has no settings, so we
  use builtin defaults).  This only has any effect if the shell
  variable `distro' is set to the empty string - so it only takes
  effect during the import.

* Set dgit-distro.foreign.cmd-git to false.  This therefore detects if
  we ever try to do the import with the default distro's `git'
  command: we are supposed to be using the one from the .dsc.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>